Function execute_query 

pub fn execute_query(
    db: &Db,
    query: &AnalyticsQuery,
) -> Result<AnalyticsResult, DbError>

Execute an analytics query against the database and return the result.

Steps:

1. Record the start time (for execution_time_ms).
2. Fetch all documents from the requested collection (O(n) copy).
3. Filter documents using the WHERE clause (if provided).
4. Compute the requested metric over the filtered documents.
5. Return the result with timing metadata.

Arguments

• db — The database handle. Used to call db.get_all(collection).
• query — The parsed analytics query (collection + metric + optional filter).